A Post-AGB Star in the Small Magellanic Cloud Observed with the Spitzer Infrared Spectrograph

We have observed an evolved star with a rare combination of spectralfeatures, MSX SMC 029, in the Small Magellanic Cloud (SMC) using thelow-resolution modules of the Infrared Spectrograph on the Spitzer SpaceTelescope. A cool dust continuum dominates the spectrum of MSX SMC 029. Thespectrum also shows both emission from polycyclic aromatic hydrocarbons (PAHs)and absorption at 13.7 micron from C2H2, a juxtaposition seen in only two othersources, AFGL 2688 and IRAS 13416-6243, both post-asymptotic giant branch (AGB)objects. As in these sources, the PAH spectrum has the unusual trait that thepeak emission in the 7-9 micron complex lies beyond 8.0 micron. In addition,the 8.6 micron feature has an intensity as strong as the C-C modes whichnormally peak between 7.7 and 7.9 micron. The relative flux of the feature at11.3 micron to that at 8 micron suggests that the PAHs in MSX SMC 029 eitherhave a low ionization fraction or are largely unprocessed. The 13-16 micronwavelength region shows strong absorption features similar to those observed inthe post-AGB objects AFGL 618 and SMP LMC 11. This broad absorption may arisefrom the same molecules which have been identified in those sources: C2H2,C4H2, HC3N, and C6H6. The similarities between MSX SMC 029, AFGL 2688, and AFGL618 lead us to conclude that MSX SMC 029 has evolved off the AGB in only thepast few hundred years, making it the third post-AGB object identified in theSMC.Comment: 4 figures, Fig. 4 color; to appear in the 20 November 2006 Astrophysical Journal Letter